Because in different countries the term "foil" and the term "film" are used to mean a web of plastic material which may be made by so-called film blowing or wide-slot nozzle extrusion, the terms are used here interchangeably.
It is known that in the production of plastic foils or film which can be wound to form a coil, thickness-error profiles can be detected for the foil across the width of which systematic plus thickness errors and minus thickness errors can be discerned.
The term "systematic" is here used to refer to errors in the thickness of the foil that arise as a function of some property of the foil-fabricating system. Such errors can develop in association with certain parts of the manufacturing system and, consequently, when a foil having such systematic errors is wound in a coil, the plus thickness errors, i.e. the thicknesses greater than the desired or setpoint value of the foil thickness, tend to accumulate in the coil to form annular bulges therein. Conversely, minus thickness errors or thickness deviations from the standard or setpoint value which are such that the thickness in a particular region is less than the setpoint value, can accumulate to form annular troughs in the coil. The system-determined thickness errors may result, for example, from tool or extrusion die tolerances or temperature differences of the extruded thermoplastic at various regions.
Such systematic thickness errors are largely unavoidable and appear in the cross section profile of the foil produced and, in a stationary state of the apparatus for producing the plastic foil, always arise at the same location across the width of the foil. If efforts are not made to avoid the build up of troughs and bulges in the coil, as described, the coil or roll has a barrel shape with annular bulges and troughs which makes the coil difficult to handle.
It has been proposed to prevent the addition of thickness errors as the foil is wound into a coil by shifting the thickness-error profile across the width of the foil and thereby substituting an error distribution for the conventional error addition. As a result, the coil can be free from error addition and thus from the bulges or troughs.
In the production of so-called blown film, in which a thermoplastified tube is generated by extruding the thermoplastified synthetic resin through an annular nozzle, the tube is then blown to form a balloon or bubble and the balloon is flattened to form a web which is wound into the coil with or without edge slitting to separate the layers of the flattened tube, either the blowing head with the extrusion tool or die must be given an angular periodic oscillating movement or the so-called flattening device must be correspondingly periodically moved. In some cases, both the blowing head and die and the flattening unit are angularly oscillated to distribute the thickness-error profiles.
These expedients have been found to involve high capital, maintenance and operating costs and thus are expensive. Furthermore, they do not always achieve the desired effect since the coil is frequently found to have irregularities even though the aforementioned bulges might not be discernible in the coil.
Another method of making such foils is to extrude the thermoplastic synthetic resin from a so-called wide slit nozzle or wide-mouth nozzle, the flat or planar foil emerging from between lips of a slot extending the width of the web and forming the nozzle or die. The thickness errors can be measured and the lips of the die adjusted to vary the extrusion gap and thereby reduce the magnitude of such errors. Such systems have been found to be costly from the measuring technology point of view. The super-position and distribution of singular thickness errors is also known, but earlier systems of this type merely reduce the thickness tolerances and do not completely eliminate the effect of thickness error addition in the coil arising from the thickness-error profile of the web.